This invention relates to a high pressure sodium vapor lamp utilizing alumina ceramic inner envelope and is particularly concerned with a high pressure sodium lamp having an inner arc tube alumina ceramic operated at a higher than normal wall temperature which improves the efficacy of the high pressure sodium lamp.
High pressure sodium vapor lamps have found widespread use during the past decade for commercial lighting applications, especially outdoor lightings. Such lamps are described in U.S. Pat. No. 3,248,590--Schmidt, High Pressure Sodium Vapor Lamps. High pressure sodium lamps typically utilize a slender tubular inner envelope of transmissive refractory oxide material resistant to sodium at high temperatures, suitably high density polycrystalline alumina or synthetic sapphire. The inner envelope typically contains a gas atmosphere comprised of sodium along with a rare gas such as xenon to facilitate starting, and mercury for improved efficiency. The ends of the inner envelope are sealed by suitable closure members affording connection to thermionic electrodes which may comprise a refractory metal structure activated by electron emissive material. The ceramic arc tube is generally supported within an outer vitreous envelope or jacket provided at one end with the usual screw base. The electrodes of the inner envelope are connected to terminals of the base, that is to shell and center contacts. The space between the inner and outer envelope is typically evacuated in order to conserve heat.
The lumens per watt or efficacy of high pressure sodium (HPS) vapor lamps may be improved by increasing the operating temperature of the arc chamber of the arc tube of the HPS lamp. HPS lamps typically have arc chambers made of polycrystalline aluminum oxide which when subjected to an operation above 1200.degree. C. causes sublimation which appreciably leads to diminished light output of the arc tube, which, in turn, reduces the lumen output of the HPS lamps. The sublimation products comprise suboxides of aluminum which deposit on the inside of the outer glass lamp jacket of the HPS lamp. Under certain conditions of lamp chemistry sodium has been observed to leave the interior of the arc chamber and form sodium beta-alumina in the wall of the alumina and permit the electrolysis of sodium. The sodium beta-alumina formations within the wall of the arc tube material during increased wall temperature operation cause the voltage of the HPS lamp to rise and also cause a light-absorbing browning of the outer jacket glass of the HPS lamp. The increase of HPS voltage and the browning of the outer jacket both contribute to decreasing the efficacy of the HPS lamp.
Accordingly objects of the present invention are to provide means for reducing the losses of a HPS lamp operated at relatively high wall temperatures contributed to by the sodium losses within the arc chamber of the HPS, and to provide such a high pressure sodium lamp having an improved efficacy.
These are the objects of the present invention and will become more apparent upon consideration of the following description of the invention.